MARCKS (Myristoylated Alanine Rich C Kinase Substrate) is the most prominent phosphoprotein substrate for protein kinase C in brain. Accumulating data have indicated a role for MARCKS in transducing extracellular signals for the regulation of actin-membrane plasticity and cellular processes associated with dynamic regulation of the neuronal cytoskeleton and synaptic restructuring, e.g., brain development and neurotransmitter signaling. Additional evidence from our laboratory has demonstrated a role for MARCKS in the hippocampus as a target for the long-term action of chronic lithium in the brain. With the recent creation of a mutant mouse model for the Macs gene which has now been backcrossed onto a C57BL/6 background to the eighth generation, MARCKS has been shown to play a critical role in brain development. Recent data from our laboratory have demonstrated for the first time that the distribution of MARCKS mRNA in brain is preferentially expressed in limbic and limbic-related regions of the brain, with a specific cellular distribution in hippocampus of both rat and mouse brain. In preliminary data, we have also demonstrated that MARCKS is expressed in a gene-dose dependent fashion in adult Macs null/+ heterozygotes, and that the approximately 50 percent reduction of MARCKS expression is associated with hippocampal mossy fiber hyperplasia, as well as enhanced spatial learning (Sections C.8b and C.9). These findings take on further significance in light of our current additional data demonstrating that DBA/2 mice which demonstrate hippocampal mossy fiber hypoplasia and significant spatial learning deficits, also show a significant elevation in hippocampal MARCKS as compared to C57BL/6 mice. This proposal outlines a three year interdisciplinary research strategy, in conjunction with the Center for Mammalian Genetics, to create mice expressing levels of MARCKS varying from 50 percent (Macs null/+) to 260 percent (MARCKS Tg+/+) of that expressed by wild-type mice. In these mice we will investigate the functional consequences of both under and over-expression of MARCKS on hippocampal mossy fiber development, hippocampal LTP electrophysiology, and spatial learning. In addition, we will assess the genetic basis of this interstrain variation in MARCKS expression and identify the positions of quantitative trait loci controlling MARCKS expression among backcross and/or F2 intercross progeny of DBA/2 and C57BL/6. Once the genetic basis of this variation is evident, we will determine if the same loci cosegregate with the length of the mossy fiber infrapyramidal projection and spatial learning in hybrid progeny expressing highest vs. lowest MARCKS. The outlined series of studies are designed to examine a direct role for MARCKS in neuroplastic events in brain related to hippocampal development and learning/memory, and assess the extent to which these complex phenotypic traits are linked to the Macs gene itself and/or interactive gene modifiers.